Reasoning: Enzymes are biological catalysts and, according to Class 9 biology, they are proteins. However, not all proteins function as enzymes.

Reasoning: The lock-and-key model states that the enzyme's active site has a fixed (rigid) shape that exactly matches the substrate, just as a key fits a specific lock.

Reasoning: As temperature increases, the rate of an enzyme-controlled reaction rises steadily due to increased kinetic energy, peaking at an optimum temperature around 40°C. Beyond this point, the reaction rate drops sharply because the high thermal energy disrupts the enzyme's structure, causing it to denature and lose its catalytic function.

Reasoning: Cofactors are non-protein substances that assist enzymes in carrying out their catalytic functions efficiently.

Reasoning: Changes in pH affect the ionization of amino acids in the enzyme, altering the shape and function of the active site.

Reasoning: The active site is the specific region of an enzyme where the substrate binds and the chemical reaction takes place.

Reasoning: Looking at the graph, a pH value of 7 sits directly within the activity curves of both enzyme Y and enzyme Z, meaning both are functional at this level. In contrast, enzyme X is completely inactive past pH 4, and neither Y nor Z shows any activity down at pH 4.

Reasoning: The diagram illustrates the lock-and-key model of enzyme action, where S is the unchanged enzyme (amylase) and Q is its specific active site where the reaction takes place. P represents the large substrate molecule (starch chain) entering the reaction, which is broken down into smaller product molecules labeled R (maltose units).

Reasoning: Glycolysis is the first stage of respiration in which one molecule of glucose is broken down into pyruvic acid.

Reasoning: ATP is formed when a phosphate group is added to ADP. This process is called phosphorylation.

Reasoning: Before entering the Krebs cycle, pyruvic acid is converted into Acetyl CoA in the mitochondria.

Reasoning: In aerobic respiration, oxygen acts as the final electron acceptor in the electron transport chain.

Reasoning: Carbon dioxide is released during the Krebs cycle when Acetyl CoA is oxidized.

Reasoning: Light reactions produce ATP and NADPH for the Calvin cycle and release oxygen as a by-product from the splitting of water.

Reasoning: Molecule IV represents sugar / glucose, which is the primary organic biomolecule produced as the end-product of the Calvin cycle during photosynthesis. The other components are inorganic: I is water (H₂O), II is oxygen gas (O₂), and III is carbon dioxide (CO₂).

Reasoning: Broad and flat leaves provide a large surface area to absorb maximum sunlight, which increases the rate of photosynthesis and food production.

Reasoning: The gas bubbles being produced by the pond weed are oxygen (O₂), which is a byproduct of photosynthesis. To produce more bubbles, the rate of photosynthesis needs to be maximized, which requires an abundance of raw materials like dissolved CO₂, high-energy bright light, and a warm temperature to optimize the activity of the plant's photosynthetic enzymes.

Reasoning: During the light reactions of photosynthesis, water molecules are split by light energy (photolysis), releasing oxygen, hydrogen ions, and electrons. The oxygen is released into the atmosphere.

Reasoning: The label Y represents NADH because it is produced across multiple stages of cellular respiration: 2 molecules during glycolysis, 2 during pyruvate oxidation, and 6 during the Krebs cycle, all heading into the electron transport chain. In contrast, X represents ATP energy bursts, and Z represents the 2 molecules of FADH₂ generated exclusively inside the Krebs cycle.

Reasoning: The carton blocked sunlight from reaching the grass. Without light, the grass could not carry out photosynthesis properly, and chlorophyll began to break down, causing the grass to turn yellow.